As there is demand for a portable electronic device having a small size and light weight, as well as high performance characteristics, lithium secondary batteries having high capacity are needed.
Graphite for a negative active material of a lithium secondary battery has a theoretical electrical capacity of 372 mAh/g. In order to obtain a negative active material with higher capacity than graphite, it is necessary to develop an amorphous carbon material or a novel material that can replace a carbon material.
For example, Japanese Patent laid-open No. Hei 5-286763 discloses coating carbonaceous material and a metal being capable of alloying an alkali metal with a carbon material.
Recently, as disclosed in Japanese Patent laid-open No. 2002-255529, there has been active development for a composite material in which Si particulates are combined to graphite particles.
However, since Si particulate included in the composite material includes a functional group such as SiOH or Si═O on its surface, such functional groups contact the electrolyte and induce slow decomposition of the electrolyte resulting in significantly deteriorated cycle characteristics. Particularly, LiPF6 as a conventional solute reacts with a hydroxyl group of SiOH to generate hydrofluoric acid (HF) easily. The HF deteriorates cycle characteristics remarkably.